The present invention relates to a vehicle having a system for controlling a downshift in an automatic transmission, and more particularly to a vehicle having a system for controlling hydraulic fluid pressure supplied to a hydraulically operated friction element to be engaged for a downshift in an automatic transmission.
Most motor vehicles have an automatic transmission including a planetary gear train for engaging the different gears. To change gears, one of the different constituent units of the planetary gear set, for example, the sun or the ring or the planet carrier is braked separately with the help of hydraulically operated friction elements, for example, the clutches or the brakes.
To keep the hydraulically operated frictional elements engaged, the hydraulic fluid under the line pressure is used. The line pressure is variable with variation in the engine load or the throttle opening degree, such that the line pressure increases in response to an increase in the engine load to avoid occurrence of slip in the hydraulically operated friction element.
For a power-on downshift, i.e., a downshift with an accelerator pedal depressed, the hydraulic fluid with a relatively high line pressure is supplied to a hydraulically operated friction element to be newly engaged. Thus, if it is subject to this high line pressure at once, the hydraulically operated friction element will engage with substantially great shock, inducing a shift shock during the downshift.
To solve this problem, it is the usual practice to provide a variable flow restrictor disposed in a hydraulic fluid supply circuit for a low clutch to be newly engaged during a downshift. Reference should be made in this regard to a variable flow restrictor in a hydraulic fluid supply circuit for a low clutch in an automatic transmission disclosed U.S. Pat. No. 4,763,545 issued to Shibayama et al on Aug. 16, 1988 (corresponding to JP 62-83539 A). The variable flow restrictor in this known automatic transmission includes an orifice and a timing valve having a position where a bypass passage in parallel to the orifice is opened. The timing valve is operable by a solenoid under control of a control unit.
If supply of hydraulic fluid to the friction element is effected through this orifice from initiation of the downshift to completion thereof, it takes a long time from initiation of engagement of the low clutch to completion thereof. Thus, the shift quality is degraded owing to shift lag or engine racing. To avoid this problem, the solenoid shifts the timing valve to the position where the bypass passage is opened upon elapse of a predetermined time after initiation of the downshift, i.e., after detection of a command for the downshift. After the bypass passage is opened, the hydraulic fluid is supplied to the low clutch quickly, thus shortening the time required for the downshift.
According to this known shift quality control system, the length of time is not variable with a variation in the engine revolution speed immediately after the downshift has been initiated, so that it is difficult to maintain a good shift quality.